The BLM estimates that there are 502 wild horses and 3,194 wild burros in Arizona, with ideal numbers less than half that. Thirty U.S. House members are urging new Interior Secretary Sally Jewell to reform the government’s wild horse management program and its spiraling budget that they say has created an “untenable situation” for both the mustangs and U.S. taxpayers.

The government's program of rounding up wild horses and burros is costly and does little to stem the growth of herds - and may actually be helping herds grow, a new report says.

The National Academy of Sciences report also said the Bureau of Land Management has flawed estimates for the number of horses and burros on federal land and that the overall herd is likely growing by 15 percent to 20 percent a year.

The report recommends alternatives ranging from contraception to "self limitation" - leaving the animals be and letting nature thin the herd through crowding and competition for food.

The BLM, which commissioned the study, said in a statement that it welcomed the findings, but wanted to consider them in depth.

The statement warned that there are "no quick or easy fixes" to the problem of managing the herds, estimated at about 31,000 horses and 6,000 burros on 179 herd management areas spread over 10 states. Almost half of the herd areas are in Nevada; about half of the burros are in Arizona.

But others called for action in light of the report.

"BLM cannot continue to spend 70 percent of the program's budget on roundups and long-term corralling," said Rep. Raúl Grijalva, D-Ariz., in a statement. "This report is a needed wake-up call that should lead to the corrective actions I've been requesting for years."

Under the "gathering" program, horses are rounded up and offered for adoption, but BLM typically finds homes for only a portion of the 8,000 animals gathered in a year. The rest are put out to pasture at government expense - about $40 million in one recent year, according to the report.

Arizona has seven herd management areas with an estimated 502 wild horses and 3,194 burros, according to BLM estimates. The study recommends that the numbers should be around 240 horses and 1,436 burros.

In a conference call last week, authors of the report said that because of the bureau's failure to properly manage the wild horse and burro ranges, the population of those animals is growing at 15 percent to 20 percent per year.

"There's also concern about the health of the animals themselves, as well as the range land," said Guy Palmer, a Washington State University professor who chaired the commission that did the study.

The report contends that no matter how much land is allocated for wild horse and burro populations, they will continue to expand beyond those boundaries.

The report said expanded use of contraceptives is one option to keep wild horse populations down.

Opponents of the BLM "gathers" cheered the findings.

"Contraception can lower herd size and prevent the terrible practice of 'warehousing' wild horses," said Grijalva's statement. "There's no reason not to start meaningful contraception efforts."

But Anne Novak, executive director of Protect Mustangs, a California nonprofit, worries such a move would be counterproductive.

"Man-made sterilization and contraceptives would domesticate horses and wipe them out. Nature should take care of it," she said. "How is a human going to look at a group of horses and decides who lives?"